1. Field of the Invention
The present invention relates to a MEMS (Micro-Electro-Mechanical Systems) capacitive sensing circuit, and more particularly, to a MEMS capacitive sensing circuit for eliminating the bias noise.
2. Description of the Related Art
The capacitive sensing device (e.g. microphone) is well known in the art and commonly used in medical equipments or communication industry. Along with the vigorous development of electrical industry and the considerable progress of manufacturing process, the microphone product is required for thinner, smaller, lighter, less power and low cost. Hence, the microphone, which can be integrated within the semiconductor manufacturing chip, is an important trend for current development.
For instance, in U.S. Pat. No. 5,577,129, a conventional ECM (electret condenser microphone) amplifier circuit is utilized to eliminate the noise generated by a bias circuit of ECM. As shown in FIG. 1, the sound wave movement causes the difference of capacity value in an ECM microphone 10, which generates a signal into the input negative node of an amplifier 12. Then the amplifier 12 amplifies and outputs the signal. Meanwhile the noise from a bias circuit 14 will enter into the input positive node and negative node of the amplifier 12 simultaneously and part of noise will be eliminated based on the common mode rejection ratio (CMRR) characteristic in the amplifier 12. However, the conventional ECM amplifier circuit requires discrete capacitors 16 and 18, and the required capacity values are between 0.1 to 10 μF, which might be too huge to design into an integrated circuit.
Additionally, the conventional MEMS microphone amplifier circuit requires a bias circuit for providing a bias voltage to MEMS microphone and the bias noise must far less than the minimum signal (several uV) outputted from the microphone. Generally, a discrete capacitor is required for stabilizing the voltage level and eliminating the noise. However, it is still a problem for designer to integrate the discrete capacitor into an integrated circuit.
Therefore, to solve the above-mentioned problems, the present invention proposes a MEMS capacitive sensing circuit for eliminating the bias noise.